1. Field of the Invention
This invention pertains to improvements in the structural body of a rotary rock bit. More specifically, this invention relates to the hydraulic function, metallurgical treatment manufacturing method, and assembly of the structural body of a rotary rock bit.
2. Description of the Prior Art
This discussion is limited to rock bits having a plurality of rotating toothed cutters which are generally somewhat conical in form. This common type of rock drill bit has not changed substantially in bodily structure in over half a century. The conical rock cutters are rotatively borne upon cantilevered journal shafts which enter the cutter bearings normal to, and central to the base of the cones. The journal shafts are directed angularly downward and radially inward relative to the centerline of the vertically cylindrical bit body. The bit body supports the journal shafts from its lower periphery, having an upper end, called the "Pin End", which is threaded for attachment to the lower end of a drill line made of pipe. The bit body also serves the function of a terminal pipe fitting to control and route a fluid flow from the drill line pipe to exit through the plurality of mud nozzles housed therein.
In use, the drill line pipe is rotated while forcing the rock bit into the earth. The rock cutter cones, with their vertices directed toward the centerline of the drill bit, roll about the centerline of the drill bit as the rock cutting teeth are forced into the geologic formation to crush and fracture it. Fluid pumped down the drill line and through the nozzles serves both to dissipate the heat of drilling, and to flush cuttings from the drilling zone and buoy them upward to the surface through the annular space between the bore hole wall and the drill line pipe.
To permit assembly of the rock cutter cones upon their respective journal shafts, the structural body of the rock bit is made in separate longitudinal segments, called "Legs", each leg incorporating one journal shaft. The segments are welded into an integral unit after being assembled with the cutters. After welding, the pin end is threaded for attachment to the lower end of the drill line.
Inventors in the art have long recognized the advantages to be realized in production of a rotary rock bit with an unsegmented body structure, yet the segmented form has remained the standard of the industry.
U.S. Pat. No. 1,388,424 issued to George in 1921 teaches the use of a unitary bit body having four conical cutters with axes nearly vertical, two being convergent and two being divergent. The cones and journals are shown integral, rotatively supported in bushings housed within the bit body. Unfortunately the cutting geometry of this design appears to be non-aggressive.
Clarence Reed, a prolific inventor in the art, describes in U.S. Pat. No. 1,636,666 and more particularly in U.S. Pat. No. 1,692,793 a two cone rock bit of conventional cutting geometry featuring a one piece rock bit body. Individual journal shafts depend from vertical posts which are mechanically drawn into bores within the bit body after assembly of the rotary cutters.
Swift and Dalldorf were granted U.S. Pat. No. 1,726,049 on a unique rock bit having three cutters with vertical axes mounted in a straight line. The cylindrical cutters carried helical teeth which intermeshed to provide mutual cleaning and synchronous rotation. The cutters depend from a one piece bit body, however, the design is both non-aggressive in cutting action and structurally weak for use in even very soft geologic formations.
U.S. Pat. No. 2,061,657 by Howard, assigned to Globe Oil Tools Company represents a notable design advance which suffered commercially from bad market timing. Two cutters depend from a one piece bit body. Near vertical journal shafts are used with strong negative camber. The upper stator end of each journal shaft is drawn into a matching locking taper within the body and secured, in the production model, by a nut on the threaded extension of the journal shaft. The patent drawing, however, depicts use of a flat drive key with a locking taper. The cutting geometry was made effective by the use of the negative camber. The Howard patent was applied for in May of 1933, but before it came to issue in November of 1936, the well known three cone bit of current commerce, U.S. Pat. No. 1,983,316 by Scott et al, assigned to Hughes Tool Company was issued, and has since preempted the marketplace.
An English inventor, Lanchester, in U.S. Pat. No. 2,648,526 teaches the use of a one piece bit body in a three cone rock bit. The independent journal shafts depend from cylindrical posts which are threadingly drawn and secured into vertically converging bores within the bit body.
A novel cutting structure using three interleaving cutters with integral journal shafts having vertically converging axes are rotatively supported by roller bearings within the one piece bit body, is described in U.S. Pat. No. 2,915,291 by Gulfelt.
The two latter designs seem never to have been commercialized.
With the advent of Electron Beam Welding, a number of patents have been issued directed to the use of this process in the production of rock bit designs having unsegmented bit bodies. U.S. Pat. Nos. 3,850,256 McQueen, 4,145,094 Vezirian, 4,158,973 Schumacher, 4,187,743 Thomas, and 4,256,194 Varel, are all illustrative of this trend. Although all of these efforts relied upon conventional prior art rotary cone cutting geometries, commercial use has not been seen. U.S. Pat. No. 4,209,124 by Baur, however, is directed to a fixture for electron beam welding a conventional segmented bit body together and is widely practiced.
U.S. Pat. No. 4,335,794 by Goodfellow shows an unsegmented bit body with an open cylindrical "Pot" formed within the lower end. Cones are mounted on journals which depend from short "Legs" which are configured to fill the pot annularly, leaving a tapered bore at the center which is then filled with a tapered plug, in turn secured by a central bolt.
Drilling hydraulics has long been a subject of general interest and study, although one with only marginal gains in practice. When drilling commences, water is pumped down the drill line and through the nozzles provided within the bit body to direct accelerated fluid streams toward the drilling zone. This fluid flow is provided for two purposes; to flush cuttings from the drilling zone and up the annular space between the drill line and the bore hole wall to the surface, and to dissipate the heat of drilling. As drilling progresses and the well bore becomes deeper, the viscosity and specific gravity of the drilling fluid must be increased in order to buoy the cuttings to the surface. Such altered fluids are known generically as "Mud". Additionally, fiberous or pulpy ingredients are added as needed to stem the loss of fluid into porous or fractured geologic formations. Such ingredients are commonly called "Lost circulation materials".
Upon returning to the surface, the mud is screened of coarse debris and then routed to a settling pond where finer debris is shed by gravity. Fluid drawn from near the surface of the pond is then used in the production of "Fresh" mud for use in continued drilling. The fresh mud contains retained fines, and when pumped down the drill line at a typical three thousand pounds per square inch pressure, will find a toehold for rapid destructive abrasive erosion in any pin-hole or crack in the drill line or rock bit body.
The conventional mud nozzle is a sharp edged orifice made of Tungsten Carbide. One such nozzle is usually provided for each rotary rock cutter and is positioned fully above the cutters relatively close to the bore hole wall, directing a high velocity fluid stream downward between cutters and radially outward against the bore hole wall. It should be noted that the stream fans out conically at a substantially high rate after leaving the nozzle.
The fluid path from the pump down the drill line is relatively free flowing until suddenly impacting the inside floor of the rock bit body. From that point the flow is very turbulent as it "cloverleafs" into coriolis circulations above each of the ports leading to the exit nozzles. In the turbulence within the bit body and in the throttling action of the mud nozzles, a drop in hydraulic pressure occurs which accounts for from fifty percent to about sixty five percent of the energy delivered by the mud pump, under favorable drilling conditions. The pressure dropped across the nozzles is sought in practice, in an attempt to reach hole bottom with the projected fluid streams.
Generous vertical channels are formed between the exterior wall of the rock bit body adjacent the nozzle locations and the bore hole wall, being provided by design to permit the free flow of fluid and entrained cuttings from the drilling zone.
Actually, the high velocity fluid cone directed across the entrance to each channel is particularly effective in blocking any fluid flow up the channel. A "hold down effect" is therefore generated which serves to keep the more substantial rock chips on the hole bottom to be ground to dust size by the rolling cutters. Ultimately, large volumes of very fine debris are forced to exit the cutting zone by passing through the wedge shaped clearance between the large end of the cutter and the bore hole wall. This action is directly accountable for the condition known as "Shale Packing", which causes the early destruction of the elastomeric seal protecting the journal bearing. Additionally, this process serves to load up the settling pond with large amounts of abrasive fines in an escalating destructive cycle. The initial step in this process, the crushing to dust of cuttings by the cutters, both impedes the penetration of the rock bit into the geological formation and abrasively wears the teeth of the rock cutters.
U.S. Pat. No. 2,901,223 by Scott, proposes a centrally located cluster of three nozzles to discharge radially outward and downward between cutters which are relatively smaller than commonly used to that they can be mutually spaced apart to avoid excessive abrasion from the nozzle discharge. Obviously this design still serves to block the entrances to the vertical chip clearance channels. This invention was intended for use in soft gummy formations.
Johnson, in U.S. Pat. No. 3,528,704 and again in U.S. Pat. No. 3,713,699 teaches the use of cavitating nozzles directly as cutting tools against the rock. A fluid stream is pulsated at high frequency and enough energy to physically vaporize the fluid in the low pressure phases of the vibratory wave. The vapor bubbles thus produced implode in the high pressure phases of the same waves, and, if very close to the rock surface, cause particles of the rock to erode away in tension. Unfortunately, cavitation is a low pressure phenomenon, and cannot be practiced in any but shallow wells.
U.S. Pat. No. 4,126,194 by Evans, is directed to the use of a tube, used in place of one nozzle, having an inlet end located on hole bottom between cutters, and having a discharge end open as a point above the discharge points of the retained nozzles. A naturally occurring differential pressure is relied upon to route cuttings from the hole bottom up the channel.
Another patent directed at drilling problems encountered in soft gummy formations, U.S. Pat. No. 3,823,789 by Garner, uses an additional nozzle located on the bit centerline, directed downward, to break up the ball that persists in forming at that location. This nozzle emits a diffused stream to avoid excessive abrasive action against the cutters. A center nozzle is current accepted practice used in conjunction with "Extended Nozzles". The extended nozzle is a short cast tubular extension which is welded into the existing nozzle port, holding in turn a very small carbide nozzle which discharges downward between cutters from about the level of the centroid of the rotary cutters. Dramatic rates of penetration are gained with this nozzle combination in certain very soft formations, however the extensions have proven to be very fragile, tending to snap off during drilling thus threatening well completion.
A very strong extended nozzle is disclosed in U.S. Pat. No. 4,077,482 issued to Ioannesian et al of Moscow U.S.S.R.. Only one nozzle is used by Ioannesian, requiring a separate special body segment to support it. Being a Russian production however, we know nothing about its field history.
Conventionally, rock bit bodies are forged, in segments, of steels such as 8610 or 4815 which are case hardening grades. After selectively case hardening, and some localized hard facing, and after the conical rock cutters are assembled to their journal shafts, the segments are welded together into an integral body. The pin end is annealed after welding to permit threading, which is the final machining operation. The massive core sections are left relatively soft intentionally because the soft steel is tough and resists fracturing. A fracture generally means that part of the bit is lost in the well bore jeopardizing well completion.
During drilling operations forces are sometimes encountered which tend to pinch the rotary rock cutters into impingement, rendering the rock bit useless because the soft metal of the body yields maintaining the pinched condition and preventing rotation of the rock cutters. The very unfortunate truth is that the forces are transient, and that most frequently they come to bear upon a new bit during its introduction to the well bore.